HtmlUnit
HtmlUnit is a “GUI-Less browser for Java programs”. It models HTML documents and provides an API that allows you to invoke pages, fill out forms, click links, etc… just like you do in your “normal” browser. It has fairly good JavaScript support (which is constantly improving) and is able to work even with quite complex AJAX libraries, simulating Chrome, Firefox or Internet Explorer depending on the configuration used.
HtmlUnit is not a generic unit testing framework. It is specifically a way to simulate a browser for testing purposes and is intended to be used within another testing framework such as JUnit or TestNG.
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